Kim Tiow Ooi

Closed-loop pulsating heat pipe

Flow visualization was conducted for the closed-loop pulsating heat pipe (PHP) using a charge coupled device (CCD). It was observed that during the start-up period, the working fluid oscillates with large amplitude, however, at steady operating state, the working fluid circulates. The direction of circulation for the working fluid is consistent once circulation is attained, but the direction of circulation can be different for the same experimental run. Phenomena such as nucleation boiling, coalescence of bubbles, formation of slug and propagation of inertia wave were observed in the closed-loop PHP. The findings showed that the meandering bends, uneven slug and plug distribution and non-concurrent boiling at the evaporator contributed to the driving and restoring forces for fluid circulation and oscillations.

Optimisation of single and double layer counter flow microchannel heat sinks

In this paper, a single layer counter flow (SLCF) and a double layer counter flow (DLCF) microchannel heat sink with rectangular channels have been modelled by employing the thermal resistance network with the correlations available in the open literature to evaluate the performance of the heat sinks. The accuracy of the prediction has been verified by comparing the results obtained with those from the more comprehensive three dimensional computational fluid dynamics (CFD) conjugate heat transfer model. Good agreements were obtained. The thermal resistance models has been linked with a multivariable constrained direct search optimisation algorithm to optimise the performance of the SLCF and DLCF microchannel heat sink under a set of design constraints. Optimisation results show that both the SLCF and DLCF heat sinks operating in the laminar out performed the heat sinks under turbulent flow conditions, both in heat transfer and hydrodynamic considerations.

Evaluation of viscous dissipation in liquid flow in microchannels

Different phenomena have been observed in various works indicating that the mechanisms of flow and heat transfer in microchannels are still not understood clearly. There is little experimental data and theoretical analysis available in the literature to elucidate the mechanisms. It is reasonable to assume that, as the dimensions of flow channels approach the micro-level, viscous dissipation could be too significant to be neglected due to a high velocity gradient in the channel. However, no evidence and analysis was presented to verify such an explanation. Therefore, in this paper, the effects of viscous dissipation in microchannel flows are analyzed and examined theoretically. A criterion to draw the limit of the significance of the viscous dissipation effects in the microchannel flows is suggested based on the present analysis.

Heat transfer study of a hermetic refrigeration compressor

Abstract This paper presents an analytical study on heat transfer and temperature distribution for a hermetic reciprocating refrigeration compressor using the lumped thermal conductance approach. In this analysis, the intricate components of a hermetic compressor were divided into 46 geometrically simplified discrete elements and each was assumed to be at a uniform temperature. The lumped thermal conductance method was then applied to all the components of the compressor to form 46 simultaneous equations, which were then solved to get the components’ temperatures. The results obtained were in good agreement with measurement. The discrepancies in the prediction lie in the assumptions made in assigning various heat transfer correlations to model the convection heat transfer effects of the fluid and solid surface boundaries, and the simplification made in distributing the various components of the compressor into discrete parts. The results of this study have been applied to actual compressor design in industry and have resulted in improved compressor performance.

A computer simulation of a rotary compressor for household refrigerators

Abstract This paper presents analytical studies of a fractional horse-power rotary refrigeration compressor and its performance comparison with measured results. The study employed a general-purpose performance model by considering the thermodynamic cycle of the compressor and its mechanical losses. The real gas equation of state is used to describe the changes of state of the refrigerant. The study reveals that for this quarter horse-power compressor unit the energy consumption in performing the compression cycle was 80–90% of the compressor shaft energy input, and the other 10–15% of the energy was dissipated as mechanical friction. The results also show that the performance predictions are satisfactory when compared with measured results. The model has been used in assisting the design of new compressors for use with new environmentally friendly refrigerants. This study also paves the way for more comprehensive simulation studies and for possible overall computerised optimisation design study in the future.

Adiabatic capillary tube expansion devices : A comparison of the homogeneous flow and the separated flow models

Abstract Literature shows that the homogeneous flow assumption has been commonly used in most of the adiabatic capillary tube modelling studies. The slip effect between the two phases was often not considered in this small diameter (about 1 mm) capillary tube. Due to the lack of experimental information of the slip ratio in the flow within the capillary tube, the more comprehensive separated two-phase flow model was not frequently used in theoretical studies. This paper attempts to exploit the possibility of the separated flow model. Comparisons of the predicted results between the homogeneous and the separated flow models are presented, together with experimental results from previous workers. The results show that the separated flow model using the Miropolskiys slip ratio combined with Lins frictional pressure-gradient correlations gives a better prediction compared to the homogeneous flow model.

Interface control of pressure-driven two-fluid flow in microchannels using electroosmosis

This paper presents an experimental investigation of the pressure-driven two-fluid flow in microchannels with electroosmosis effect. Two fluids, aqueous NaCl solution and aqueous glycerol, were introduced by syringe pumps to flow side by side in a straight microchannel. The external electric field was applied on the NaCl side. Under the same inlet volumetric flowrate condition, the applied electric field was varied. The interface position between the two fluids with the electroosmosis effect was studied in the first part of the experiment using the fluorescence imaging technique. In the second part, the velocity field was measured using the micro-PIV technique. The parameters, flowrate and electric field which affect the interface position and velocity field were investigated. The measured velocity field from the experiment agrees well with that of theoretical analysis.

Electro-osmotic control of the interface position of two-liquid flow through a microchannel

This paper presents theoretical and experimental investigations of the pressure-driven two-liquid flow in microchannels with the AN electro-osmosis effect. For fully developed, steady state, laminar flow of two liquids under the combined effects of pressure gradient, electro-osmosis and surface charges at the liquid–liquid interface, we have derived analytical solutions that relate the velocity profiles and flow rates to the liquid holdup, the aspect ratio of the microchannel, the viscosity ratio of the two liquids and the externally applied electric field. It was shown that adjusting the externally applied electric field could control the fluid interface position precisely. The prediction from the proposed model compares very well with measured data.

A study of multiple heat sources on a flat plate heat pipe using a point source approach

Abstract An analytical approach has been employed to study liquid flows in an isotropic wick structure of a flat plate heat pipe with multiple heat sources. In this study, the heat sources have been modeled as point sources using the Dirac Delta function to describe the heat distribution. The two-dimensional pressure and velocity distributions are shown and discussed. The study has been extended to locate the positions of the multiple heat sources for optimum heat pipe performance. The optimum performance of the heat pipe is accomplished when the minimum pressure drop is attained across the wick structure.

Frequency-dependent velocity and vorticity fields of electro-osmotic flow in a closed-end cylindrical microchannel

The frequency-dependent electro-osmotic flow in closed-end cylindrical microchannels is analyzed in this study. A dynamic ac electro-osmotic flow field is obtained analytically by solving the Navier–Stokes equation using the Green function formulation in combination with a complex variable approach. Onsagers principle of reciprocity is demonstrated to be valid for transient and ac electro-osmotic flow. The effect of a frequency-dependent ac electric field on the oscillating electro-osmotic flow is studied. The induced pressure gradient is analyzed under the effects of the channel dimension and the frequency of electric field. Based on the Stokes second problem, the solution of the slip velocity approximation is presented for comparison with the results obtained from the analytical solution developed in this study. In addition, the expression for the electro-osmotic vorticity field is derived, and the characteristic of the vorticity field in ac electro-osmotic flow is discussed.